Hi,
Here is how you can do it:
1) On your host PC run "sudo apt-get install android-tools-fastboot" to install the fastboot utility;
2) Connect your NanoPi M3 to your host PC via a serial cable (e.g. PSU-ONECOME). Press the enter key within two seconds right after you power on your NanoPi M3 and you will enter uboot's command line mode;
3) After type in "fastboot" and press "enter" you will enter the fastboot mode;
4) Connect your NanoPi to this host PC via a microUSB cable and type in the following command to flash u-boot.bin;
fastboot flash bootloader xxx.bin
Warning: you cannot update this SD card by commanding "dd". This command will cause trouble when booting the NanoPi .